1. Field of the Invention
The present invention relates generally to the lithium niobate substrates, and more particularly to a lower cost surface acoustic wave (SAW) substrate made from lithium niobate grown from a raw material containing Ta (Ta) impurities.
2. Description of the Related Art
Niobium pentoxide (Nb2O5) powder prices are a substantial part of the raw material cost for LN. Existing suppliers of Nb2O5 powders for crystal growth typically refine this powder from columbite/tantalite concentrate. The ore is mostly mined in Australia and central Africa. Most refiners today are dissolving the concentrate in HF, the refractory fluorides are then dissolved in an organic solution, and a solvent extraction process is used for separating Nb from Ta. An alternative distillation process to separate these metals is used in the chlorination process, typically less economic for columbite/tantalite concentrate. (Eckert, Reichert et al. in Niobium, proceedings of the international symposium Niobium 2001, p 67, 2001).
Because of the demand for high purity Ta in capacitor manufacturing, the Nb2O5 can be produced as a byproduct of the liquid extraction process mainly geared towards Ta production. With modest demand for Nb metal or Nb2O5 powders, the supply used to exceed the demand and price levels were reasonably low. More recently, the demand for high purity Nb2O5 powder has increased, mostly because of applications in optical lens manufacturing phasing out lead containing glasses. This, together with possibly other market factors puts upward pressure on Nb2O5 powder prices.
Currently, Nb2O5 powders considered adequate for LN growth have purity ranging from 99.99% (4N) to 5N (Suzuki, T. (1995). “The elimination of defects in Czochralski grown lithium niobate.” Journal of Materials Science 30(11): 2873-6). This enables pullout efficiencies of 85% or more (Bordui, P. F., R. G. Norwood, et al. (1991). “Compositional uniformity in growth and poling of large-diameter lithium niobate crystals.” J. Crystal Growth 113: 61-68.) Assuming a powder at the low purity range (4N) and the only impurity occurring is Ta, then an estimate of an upper bound for Ta concentration of 100 ppm, as a worst case scenario for powders used in prior art growth of high quality lithium niobate, is made.
To reduce SAW wafer cost and decrease the price dependency on the current source of powders, lower cost alternatives for the niobium used for high volume LN growth is needed. Cost however is not the only criterion. The SAW properties need to be well controlled to satisfy the stability requirements of the SAW device manufacturers, and the grown crystals need to have adequate melt fraction conversion to make good use of the growth equipment. Center frequency stability, directly related to SAW velocity, is important for producing high yield in high performance SAW filters.